Human gene mapping has become an integral component of contemporary medical genetic research. Two decades ago the number of assignments was limited and restricted to certain blood group antigens, enzyme polymorphisms and X-linked morphological traits. Today 5,000-10,000 genes, fragile sites, and polymorphic DNA markers have been placed upon the gene maps. Three types of maps are currently used: cytogenetic, linkage and physical. The cytogenetic map typically is used to assign known genes to positions on chromosomes. This is very useful in cancer genetics where information exists regarding cytogenetic aberrations in tumors. Linkage mapping requires the analysis of co-segregation of polymorphic loci in large numbers of meiotic products. This type of map is used in the positional cloning approach to gene isolation where families segregating particular disease phenotypes are available for study. Physical mapping refers to the construction of high resolution molecular contigs utilizing specialized cloning vectors. This is the final step in disease gene isolation and characterization where a piece of DNA containing the genetic material of interest is examined at the sequence and functional levels. The Genetics Section maintains a vigorous gene mapping endeavor where contributions are made to all three types of maps. These assignments to the human gene map advance the fundamental biological knowledge of the human and mammalian genome, localize functional genes that may be considered as candidate loci for disease phenotypes, and provide linkage information that may be used in positional cloning strategies for disease gene identification.